The present invention relates to optical communication systems and more particularly to systems and methods for optical amplification.
Dense wavelength division multiplexing (DWDM) systems are evolving to both increase the distances over which DWDM signals may travel without regeneration and also to expand data carrying capacity by increasing the number of channels. To support this evolution, DWDM amplification technology is increasingly relying on Raman amplifiers including distributed Raman amplifiers (DRAs).
To further enhance the performance of Raman amplification, it is desirable to use both counter-propagating and co-propagating pump signals. See U.S. patent application Ser. No. 09/899,872, the contents of which are herein incorporated by reference in their entirety for all purposes. The use of co-propagating pumping substantially improves OSNR performance. However, the use of co-propagating pumping may also give rise to undesirable effects such as non-linear effects, Raman crosstalk, and RIN pump noise. In particular, four-wave mixing (FWM) crosstalk is a key nonlinear impairment in WDM transmission systems, especially those employing close channel spacing.
With the introduction of co-propagating pumping for Raman amplification, the four-wave mixing products are generated not only by the interaction among the closely spaced WDM channels but also by the interactions between multiple pump modes and WDM signals. Consider that a co-propagating pump source, although ideally narrowly concentrated at a single spectral peak, may actually include several spectral peaks corresponding to different cavity modes. Four-wave mixing interaction can then occur between all WDM channels and WDM pump modes. Depending on fiber chromatic dispersion and pump power, the resulting impairment can make use of co-propagating Raman pumping impractical.
What is needed are systems and methods to obtain the benefits of the use of co-propagating pumping in Raman amplification without incurring intolerable levels of four-wave mixing crosstalk between WDM channels and pump signals.
By virtue of one embodiment of the present invention, four-wave mixing crosstalk between co-propagating Raman amplification pump sources and WDM channels is suppressed. Therefore, co-propagating pumping may be applied to Raman amplification without incurring penalties due to four-wave mixing crosstalk. Pump power need not be substantially increased to accommodate use of the present invention. Also, the use of lower cost Fabry-Perot pump sources is facilitated. In one implementation, the advantageous suppression of four-wave mixing crosstalk between pump signals and WDM channels is accomplished by imposing chromatic dispersion on the co-propagating pump sources.
A first aspect of the present invention provides a method for operating a Raman pump unit to suppress four-wave mixing interaction between pump modes and data signals. The method includes: generating a pump signal and applying a wavelength-dependent phase shift to the pump signal.
A second aspect of the present invention provides a Raman pump unit including: a pump signal source that outputs a pump signal and a dispersion application unit that introduces a frequency-dependent phase shift to the pump signal.
A third aspect of the present invention provides a Raman amplification system. The Raman amplification system includes a Raman pump unit that generates a pump signal and a fiber into which the pump signal is injected in a direction of propagation of a signal to be amplified. The Raman pump unit includes: a pump signal source that outputs the pump signal and a dispersion application unit that introduces a frequency-dependent phase shift to the pump signal.